The development of electrochemical sensors and actuators has been a recent area of study, with preliminary advancements in artificial muscle and micro-electromechanical systems (MEMS) technologies. An electrochemical actuator is defined as a device that converts synergistic electrical and chemical processes to produce a mechanical response. The mechanical response typically resembles a bending or flexing of the actuating material, resulting from the electrochemical excitation.(1) These devices have potential applications in medicinal procedures like arterial bypass surgery, where delicate catheter insertion and manipulation is critical to success.
Other developments for medicine may come from microactuators being applied to artificial muscle research. Another field that is optimistic for actuator development is optics, where microactuators may be employed for positioning of mirrors or lenses. Additionally, it has been proposed that aircraft will be constructed with “microflaps” that improve drag properties for enhanced landing.(2) Applications to MEMs and the new proposed NEMs devices are also quite significant. Regardless of the many scientific applications for actuators/microactuators, it is imperative for researchers to evaluate and characterize the necessary properties that allow for successful technological integration.
Studies designed to evaluate actuation properties specific to conducting polymers, carbon nanotubes, and ion-exchange polymer metallic composites (IPMCs) have been reported by many groups in the literature.(2-6) The most prominent conducting polymers are polypyrrole(3-4) and polyacetylene.2 Controlled variations of redox reactions on the polymer backbone account for the observed actuation.(2-4) Work done by Baughman et. al., has initiated the analysis of carbon nanotubes as actuating devices, operating via a non-Faradaic mechanism.(5) Another very popular area for thin film polymeric actuators has been the analysis of metal-doped perfluorocarbonsulfonic acid ionomer actuators (usually NAFION®, produced by ALRDICH, Milwaukee, Wis.), which function via ion intercalation and osmotic effects.(6) Although each of these systems operates under different conditions and mechanisms, the overall goal is to produce a highly efficient actuator for device application.
Electrochemical Actuators
Construction of electrochemical actuators consists of three components: anode, cathode, and electrolyte. Ionic species are transported through the electrolyte to establish electric double layers, producing non-faradaic processes. The charge transfer initiates dimensional changes in the actuator electrodes, producing axial strain (Length/Length), and a corresponding macroscopic deflection. Electrochemical actuators are different than other actuator types like electrostatic and piezoelectric, since an applied voltage stimulates the operation rather than an electric field. This provides a significant advantage for electrochemical actuators since only a few volts and small currents are needed to produce work responses that are an order of magnitude greater than electrostatic or piezoelectric ones.(2) 
Electrochemical actuators satisfy many of the ideal properties necessary for development of artificial muscle or for utilization in MEMS devices, but currently many such systems contain several disadvantages. Shahinpoor highlights several key factors which are required for micromanipulation: (1) flexible material, (2) long cycle life, and (3) fast, simple reaction mechanism.(7) More specifically, actuator electrodes should be thin, with minimal inner electrode separation, to promote higher rates of diffusion for ionic intercalation. Also, surface effects should minimize electrode resistance to enhance electrical stimulus. Electrolyte selection requires ionic species, which have high mobility to maintain fast transfer rates between electrodes in the faradaic actuators. Baughman has cited these characteristics as dominating factors in the large work per cycle and force values obtained for conducting polymer actuators and polymeric gel actuators, respectively.(5) While electrochemical actuators exemplify many key features, there are several limitations that require attention. Most importantly is the necessity of an electrolyte solution, discouraging any attempts for solid-state structures. Some attempts however, are being directed towards use of a solid electrolyte, like hydrated poly(vinyl alcohol)/H3PO4.(2) A second observed shortcoming is the limited cycle life of the electrochemical actuators largely attributed to the doping/de-doping effects of ion intercalation for faradaic actuators.(2) A major disadvantage shown for IPMC actuators is delamination of the surface metal coating such as gold on polypyrrole(2) or platinum on poly(acrylonitrile)(8) that is required for actuation at high voltages. While these characteristics outline important guidelines for actuator development, consideration of several types, namely conducting polymer, gel fiber and ionic conducting polymers (ionomers), can in turn provide further understanding.
Substantial work has been done to evaluate the electrochemical properties of gel, conducting polymer, and IPMC systems. Polyelectrolyte gels are unique in that under an electric field, they swell in volume inducing a mechanical change. A recent report using poly(acrylonitrile) (PAN) gel fibers exhibited swelling when exposed to alkaline media and contraction during acidic conditions. These gel fibers coated with platinum or intermixed with high doping of graphite fibers, produced an approximately 40-50% elongation after 10 minutes of electrical activation.(8) These values are considerably higher than polythiophene, which was shown to expand by only 2% after 20 minutes.9 Although the PAN gel systems produced considerable strain, slow reaction times and high voltages (10-20 V excitation) display inferior properties to other actuator systems.(8) Better performance systems have been observed with conducting polymers like polypyrrole(3,4) and polyacetylene.(2) These conducting polymers operate through redox reactions, causing length changes in the carbon-carbon bonds on the polymer backbone. Electrolyte and charge density effects have also been cited as potential mechanistic factors for conducting polymer actuators.(3) Film expansion values for these films have been reported to equal 3% for polypyrrole(4) at neutral pH and 1.6% for alkali-doped polyacetylene films. While the ionic dopant species has been shown to be a critical parameter during actuation, solvent effects are reported to be just as significant. Osmotic effects have been proposed to significantly alter the degree of actuation in conducting polymers by increasing film expansion upon decreased electrolyte concentration. Bay et. al., attribute this result to the influx of water into the polymer films from the high ionic strength of the doped polymer matrix.(3) Our results with the nanotube composite actuators supports the significance of the internal electrochemical reactions at the polymer backbone and shows that normalization of electrolyte concentrations is critical for comparison of different conducting polymer systems.(3) 
While attempts to develop actuators using conducting and gel polymers have been realized, increased response rates, higher cantilever deflection limits, and lower operating voltages have been characteristically shown for the ionic polymer NAFION®. The perfluorinated ionomeric polymer, NAFION® has been shown to actuate under electrical stimulus in an electrolyte solution when doped with large >40% w/w amounts of platinum. The prominent structural model for NAFION® considers a hydrophobic region with the fluorocarbon backbone and interstitial channels with ionic clusters that contain sulfonate groups. The micellular-like ionic clusters have sulfonate groups with bound H2O and counter ions, usually H+ or Na+. While the interstitial channels have some H2O present, mostly it is void volume. Variations in morphology for these locations, the ionic clusters and interstitial channels, is directly affected by the cation charge bound to the sulfonate. Hence movement of ions within the polymer can alter the void volumes, and by design cause a constriction/expansion of the polymer—that can be characterized as actuation.
Actuation using NAFION® in aqueous electrolytes has been performed upon surface metal doping, typically platinum or gold, at concentrations >3 mg/cm2.(58) Abe et. al., first showed the effects of counter cation species in the electrolyte, concluding that Li+ produced the largest displacement, ˜1 mm towards the anode.(6) The mechanism provided for this electrochemical actuation supports the bending and relaxation processes observed. Upon electrical excitation (a few volts or less), the hydrated counter cation moves to the cathode side of the electrode, swelling the composite with water, and inducing a bend in the film towards the anode. After prolonged electrical excitation of the film, there is an observed relaxation or creeping process back towards the cathode side. Osmotic effects have been cited as the source of this occurrence. Therefore, the presence of water has an astounding impact on the overall mechanism.(6) 
Improvements to the actuation process, namely enhanced load capability, larger displacements, and surface modifications, have been the focus of recent work.(11,12) Film thickness effects on load capability have shown that doubling the thickness for each NAFION® film corresponds to an 8-fold increase in force generated.(11) While commercially available films are ˜200 μm, Shahinpoor et. al., have developed a procedure to produce 2 mm thick NAFION® films.11 Another group has evaluated the metal doping characteristics of NAFION® films by roughening the surface to increase surface area sufficient to enhance platinization for an observed 50% increase in displacement.(12) While platinum has been the traditional metal to dope NAFION®, other reports have investigated effects of gold,(13,14) and copper-platinum,(15) as superior dopants. Although the surface effects have been studied in considerable detail, significant effort has also been made towards understanding the H2O and ionic conductivity effects within the NAFION® matrix. NAFION® has been shown to have both bound and non-bound water present, with the amounts and ratios varying with the type of counter cation present. The ability to hydrate the counter cation, especially as it migrates during electrochemical actuation, is a property distinct to each cation. Electrolytes with lithium cations have been shown to exhibit the best actuation properties. This can be attributed to the high ionic conductivity16 and large hydration spheres observed for Li+ in the NAFION® matrix.(6) Although promising results have been shown for NAFION®-based actuators, the necessity to have ion intercalation at the cathode introduces a time deficiency for migration, similar to the gel and conducting polymer systems.
Carbon Nanotube Actuators
Demonstration of carbon nanotube actuators in 1999 by Baughman et. al., displayed a novel material for conversion of electrical energy into mechanical energy. Preliminary results predicted the highest work capacity of known actuating materials.(5) The single wall carbon nanotube devices were constructed using strips of “buckey paper” sheets assembled from as-produced laser generated material. Each strip was placed on opposite sides of an insulating piece of double-stick tape and the carbon nanotube electrodes were connected to clamped platinum electrical leads.
The device was inserted in a 1M NaCl solution and several volts were applied, producing deflection at the cantilever tip up to about a centimeter. Further characterization showed that the bimorph cantilever actuator produces strain results which are >0.2%. This value already exceeds the highest strain value of ˜0.1% known for ferroelectric ceramics. Theoretical predictions indicate that carbon nanotube actuators can potentially reach strain values of ˜1% when the carbon nanotubes exist as individual single walled nanotubes (“SWNTs), i.e. not bundled. The initial work showed surface area measurements using the Brunauer-Emmett-Teller (BET) method of the buckeypapers to be ˜300 m2/g, whereas individual SWNTs are predicted to have values of ˜1600 m2/g.(5) This correlates with the five-fold decrease of observed strain versus the theoretical value based on surface area assignments. A result that is explained by only the exterior SWNTs in the bundles participating in the actuation highlighting an important observation to support the mechanism for actuation. It was concluded that the actuation in carbon nanotubes is caused by charge injection from the electrolyte salt present in solution, promoting an electric double layer.5 
In turn, the charge induces quantum mechanical expansion of the covalent framework of the carbon nanotubes, producing an observed strain. Therefore, a larger degree of actuation was proposed for debundled SWNTs, resulting from an increased surface area, thereby enhancing electric double layer effects. Overall, the initial study by Baughman et. al., has displayed the extreme potential for carbon nanotube actuators and provides a preliminary understanding of the mechanism which supports this technology.(5) 
There have been a number of recent reports describing potential applications of carbon nanotube actuators. An example is the pneumatic response for carbon nanotube buckey papers due to gas formation between carbon nanotube layers, also shown by Baughman et. al.(17) Results indicate >2% contraction of the carbon nanotube sheets when exposed to a 5M NaCl solution and voltages between +0.5 and +1.5 V. Although these results are impressive, the requirement to develop consistent carbon nanotube paper layers to form reproducible pneumatic actuators with long life cycles is limiting.(17) Other attempts to study the actuation properties of individual SWNTs has been fraught with difficulties, although some success has been achieved growing carbon nanotubes across pre-trenched wafers using the CVD process, and monitoring potential actuation using an Atomic Force Microscopy (AFM) tip.(18) Additional attempts are being made to develop “nanotweezers,” where individual SWNTs bound to an AFM tip under an applied dc voltage cause the two carbon nanotubes to approach each other. Such techniques may be applicable to the development of manipulating structures for nanoelectronics.(19) 
Each type of carbon nanotube, single wall or multi-wall, exhibits unique and useful properties for both basic science and applied technology. Although SWNTs are the focus of this invention, multi-wall carbon nanotubes (MWNTs) are also easily applied to the system
Carbon Nanotube-Polymer Composites
A new area of research is concentrating on developing carbon nanotube-polymer composites. The goal is to develop novel materials exhibiting unique electrical, thermal, optical, and mechanical properties. Some research has been devoted to developing composites that successfully transfer load requirements from the polymer backbone to the dispersed carbon nanotubes. Ideal for such applications, carbon nanotubes exhibit a large aspect ratio, which is conducive to formation of a network within the polymer matrix. The established network at high doping levels can lead to a percolation threshold, which is proportional to the aspect ratio, ˜103 for SWNTs.(20) Since the tensile strength of carbon nanotubes has been predicted to be 10-100 times that of steel, SWNT polymer composites could significantly enhance the strength of composite materials. Initial studies focused on MWNT-polystyrene composites for load transfer effects, concluding that 1% w/w dispersions significantly increased the elastic modulus and break stresses of the resulting composites by ˜40% and ˜25%, respectively.(21) Although the results are encouraging, the carbon nanotube material used in the above study(21) was as-produced MWNTs, containing metal catalyst impurities and numerous structural defects. This preliminary work has perpetuated more in depth analyses, including the recent proposal that SWNTs present superior load transfer abilities over MWNTs, since the polymer chain is interacting directly with the outermost shell responsible for carrying the imposed load.(22) When using MWNTs, the internal shells are free to move and are only stabilized by weak Van der Waal's forces, thereby allowing eventual deterioration of composite strength. However, to develop successful composites using SWNTs, researchers have proposed the necessity to overcome the bundling effects exhibited by SWNTs. Ajayan et. al., have calculated the force required to separate SWNTs individually from the bundles. Results indicate that energetically it would be easier for a polymeric matrix to pull a SWNT from the end of the bundle, rather than normal to the bundle, overcoming the carbon nanotube's Van der Waal's interactions for the bundle. Depending on the interfacial strength between the carbon nanotube and polymer matrix, this force normal to the bundle may result in shearing of the SWNT, specifically at defective sites along the sidewall of the carbon nanotube.(23) 
While load transfer composites represent one area for carbon nanotube-polymer composite applications, another significant approach utilizes the high thermal conductivity of carbon nanotubes. Since thermal transport in solid structures results from a combination of conduction by electrons and phonons, successful interaction between polymer matrices and carbon nanotubes can be used for thermal management. Since theoretical predictions for thermal conductivity in SWNTs equal 6000 W/mK,(20) the integration of SWNTs into polymer composites shows extreme potential for these applications. A recent report has shown a 125% increase in thermal conductivity for a 1% w/w as-produced SWNT-poxy composite.(20) In addition, molecular modeling has provided certain theoretical insights to help support the observed carbon nanotube polymer interactions. Gong et. al., have reported an increase in Tg of 25° C., and an elastic modulus increase of 30% for epoxy composites with 1% w/w doping of carbon nanotubes.(24) 
Carbon nanotube-polyethylene composites have exhibited a higher glass transition temperature, Tg, than the native polyethylene. These results have shown an increase in thermal expansion coefficients for composite systems, explained by the rigidity of dispersed carbon nanotubes with uniform phonon vibrations, preventing polymer rearrangement at higher temperatures.(25) A similar report using carbon nanotube-polystyrene composites shows enhanced interaction between polymer and carbon nanotubes due to a thermal expansion coefficient mismatch, which is proposed to increase the interlocking mechanism when composites are cooled from their melt.(57) A need exists for the integration of carbon nanotubes with polymer matrices.
Carbon Nanotube-Polymer Composite Actuators
Successful integration of high purity SWNTs with a polymer matrix to develop novel devices is at the forefront of many research groups. Introduction of highly conductive SWNTs into the NAFION® polymer could significantly enhance the polymer film conduction, the composite tensile strength and thermal stability, which may improve the conditions for actuation.